1. Field of the Invention
This invention relates to a process for extracting hydrocarbons from the earth. More particularly, this invention relates to a method for recovering especially viscous hydrocarbons, e.g. bitumen, from a subterranean formation using at least two wells for injection and production, and which includes critical manipulative steps with heated fluid.
2. Description of the Prior Art
In many areas of the world, there are large deposits of viscous petroleum, such as the Cold Lake, Athabasca and Peace River regions in Canada, the Jobo region in Venezuela and the Edna and Sisquoc regions in the United States. These deposits are generally called "tar sand" and "heavy oil" deposits due to the high viscosity of the hydrocarbons which they contain and may extend for many miles and occur in varying thickness of up to more than 300 feet. Although these deposits may lie at or near the earth's surface, generally they are located under a substantial overburden which may be as great as several thousand feet thick. Tar sands located at these depths constitute some of the world's largest presently known petroleum deposits.
The tar sands contain viscous hydrocarbon material, commonly referred to as bitumen, in an amount which ranges from about 5 to about 20 percent by weight. Bitumen is usually immobile at typical reservoir temperatures. For example, at reservoir temperatures of about 48.degree. F., bitumen viscosity frequently exceeds several thousand poises. At higher temperatures, such as temperatures exceeding 200.degree. F., bitumen generally becomes mobile with a viscosity of less than 345 centipoises.
Since most tar sand deposits are too deep to be mined economically, a serious need exists for an in situ recovery process wherein the bitumen is separated from the sand in the formation and recovered through production means, e.g. wells drilled into the deposit.
In situ recovery processes known in the art include emulsification drive processes, thermal techniques (such as fire flooding), in situ combustion, steam flooding and combinations of these processes.
Any in situ recovery process must accomplish two functions: (1) the viscosity of the bitumen must be reduced to a sufficiently low level to mobilize, e.g. fluidize, the bitumen under the conditions prevailing; and (2) sufficient driving energy must be applied to that treated bitumen to induce it to move through the formation to a production well.
As previously noted, among the various methods that have been proposed for recovering bitumen in tar sand deposits are heating techniques. Because steam is generally the most economical and efficient thermal energy agent, it is clearly the most widely employed.
Several steam injection processes have been suggested for heating the bitumen. One method involves a steam stimulation technique commonly called the "huff and puff" process. In such a process, steam is injected into a well for a certain period of time. The well is then shut in to permit the steam to heat the oil. Subsequently, formation fluids, including bitumen, water and steam, are produced from the well. Production is later terminated and steam injection is preferably resumed for a further period. Steam injection and production are alternated for as many cycles as desired. A principle drawback to the "huff and puff" technique is that it does not heat the bulk of the oil in the reservoir and consequently reduces the oil recovery.
Another method of recovering viscous petroleum materials from subterranean formations is through the use of thermal drive techniques. Typically, thermal drive techniques employ an injection well and a production well which extend into the reservoir formation. In operation, a hot fluid (usually steam) is introduced into the formation through the injection well. Upon entering the formation, the hot flowing fluid lowers the viscosity of the petroleum materials therein and subsequently drives the lower viscosity fluid to a production well.
It has been found that conventional thermal drive processes generally are not commercially effective in recovering bitumen from tar sands. The basic problem in high viscosity hydrocarbon formations, such as tar sands, is restricted fluid mobility in the reservoir. One reason for this is that the bitumen tends to cool and increase in viscosity as it moves away from the injection well where the steam or hot fluid is most effective. Once the bitumen attains a high enough viscosity, it banks up and forms an impermeable barrier to further flow toward production wells.
Another problem with steam drive is that the driving force of the steam flooding technique is ultimately lost when breakthrough occurs at the production well. Steam breakthrough occurs when the steam front advances to a production well and steam pressure is largely dissipated through the production well. Fluid breakthrough causes a loss of steam driving pressure characterized by a marked diminuation in the efficiency of the process. After steam breakthrough, the usual practice, as suggested in U.S. Pat. No. 3,367,419 (Lookeren) and U.S. Pat. No. 3,354,954 (Buxton), is to produce without steam drive until further steam injection is necessitated or production is terminated.
U.S. Pat. No. 3,259,186 (Dietz), for example, appears to have an early teaching of conventional "huff and puff." The patent discloses a method for recovering viscous oil from subterranean formations by simultaneously injecting steam into several adjacent injection wells to heat the formation. Formation fluids are then produced from the injection wells. After several cycles, steam drive can be established by injecting steam into one injection well while using another for production. U.S. Pat. No. 3,280,909 (Closmann, et al) discloses a conventional steam drive comprising steam injection to produce interconnecting fractures, but insufficient to produce oil, followed by steam drive at conventional pressures and rates.
Several variations of steam stimulation have been tried, each with its own distinct sequence of steps. U.S. Pat. No. 3,796,262 (Allen, et al) teaches a method of injecting steam at a rate greater than the production rate but less than the rate needed to fracture the formation. The injection is stopped when live steam breaks through to the production well, but production continues at a high rate until the pressure drops.
U.S. Pat. No. 4,182,416 (Trantham, et al) discloses a method of pattern injection and production wherein steam is injected at the injection wells until it breaks through to one of the production wells which is then shut-in while injection continues. Later, the injection well communicating with the production well is shut-in, and the production well is produced for a period of time.
Also, U.S. Pat. No. 4,130,163 (Bombardieri) teaches a method of simultaneous injection of steam into the injection and production wells. After the hydrocarbons are sufficiently mobilized, the injection well is shut-in, and the production well is opened. Finally, steam is again injected into the injection well, but at a restricted rate, to help drive the oil to the production well.
While all of the above methods are of interest, the technology has not generally enabled cost effective recovery of oil for commercial development of tar sands. There is a continuing need for an improved thermal system for effectively recovering viscous hydrocarbons from subterranean formations such as tar sand deposits.